FIG. 8 is a perspective view illustrating a conventional semiconductor visible laser diode. In the figure, there are successively disposed on an n type GaAs substrate 101 an n type AlGaInP cladding layer 102, an undoped GaInP active layer 103, a p type AlGaInP cladding layer 104, and a p type GaAs cap layer 105. A ridge structure includes the p type GaAs cap layer 105 and a part of the p type AlGaInP cladding layer 104. An n type GaAs current blocking layer 106 is disposed on the p type AlGaInP cladding layer 104 contacting side surfaces of the ridge. A p type GaAs contact layer 107 is disposed on the p type GaAs cap layer 105 and on the n type GaAs current blocking layer 06. An n side electrode 108 is disposed on the rear surface of the substrate 101 and a p side electrode 109 is disposed on the contact layer 107.
A method for manufacturing the laser diode of FIG. 8 is illustrated in FIGS. 9(a)-9(e). In these figures, the same reference numerals as in FIG. 8 designate the same or corresponding parts.
Initially, as illustrated in FIG. 9(a), the n type AlGaInP cladding layer 102, the undoped GaInP active layer 103, the p type AlGaInP cladding layer 104, and the p type GaAs cap layer 105 are successively grown on the n type GaAs substrate 101 by metal organic chemical vapor deposition (MOCVD). The MOCVD growth is carried out at a temperature of 675.degree. C. and a pressure of 150 Torr. Zinc (Zn) is employed as the p type impurity dopant of the p type AlGaInP cladding layer 104 and the p type GaAs cap layer 105. After the crystal growth, the substrate is cooled in an arsine (AsH.sub.3) atmosphere to prevent As atoms in the uppermost p type GaAs cap layer 105 from escaping.
In the step of FIG. 9(b), an SiN film 115 is formed on a center part of the p type GaAs cap layer 105. Using the SiN film 115 as a mask, the structure is selectively etched to form a ridge (FIG. 9(c)).
Thereafter, the n type GaAs current blocking layer 106 is grown on the p type AlGaInP cladding layer 104 by MOCVD (FIG. 9(d)). Since no crystalline material is grown on the SiN mask 115, the current blocking layer 106 is selectively grown on opposite sides of the ridge, so that the ridge is embedded in the current blocking layer 106.
After removing the SiN mask 115, the p type GaAs contact layer 107 is grown by MOCVD (FIG. 9(e)). Since the GaAs layers 105 and 106 are exposed over the surface of the structure after the removal of the SiN mask, the growth of the p type GaAs contact layer 107 proceeds forming a flat surface which facilitates contact with a heat sink or an electrode. After the growth of the p type GaAs contact layer 107, the substrate is cooled in an arsine atmosphere to prevent As atoms in the p type GaAs contact layer 107 from escaping, whereby an even surface of the p type GaAs contact layer 107 is attained.
To complete the laser structure of FIG. 8, the n side electrode 108 is formed on the rear surface of the substrate 101 and the p side electrode 109 is formed on the contact layer 107. Usually, the n side electrode comprises Au/Ge/Ni and the p side electrode comprises Ti/Au.
In order to attain the desired laser characteristics of the semiconductor laser diode as shown in FIG. 8, the carrier concentration in the p type AlGaInP cladding layer 104 must not be less than 5.0.times.10.sup.17 cm.sup.-3. A low carrier concentration in the cladding layer 104 causes an increase in the laser oscillation threshold and poor output characteristics during the high temperature operation. In the conventional production process illustrated in figures 9(a)-9(e), sufficiently high carrier concentration in the p type AlGaInP cladding layer 104 is achieved by increasing the Zn/III ratio, i.e., the flow rate ratio of the Zn source gas to the source gas of the group III material, to 0.7-1.0 during the growth of the p type AlGaInP cladding layer 104.
In the laser diode manufactured as described above, however, the carrier concentration in the p type AlGaInP cladding layer 104 significantly varies, whereby the laser oscillation threshold current varies and the output characteristics during high temperature operation are very poor. The variation in the carrier concentration occurs because the Zn dopant added to the AlGaInP layer is not completely activated and the activation ratio varies.
In Journal of Crystal Growth 118 (1992), pp. 425 to 429, hydrogen in an AlGaInP layer is reported as a main cause of the reduction in the activation ratio of the Zn dopant atoms in the AlGaInP layer. More specifically, during cooling the substrate in the arsine atmosphere after the growth of the p type GaAs contact layer, arsine (AsH.sub.3) is decomposed and generates hydrogen ions. This hydrogen enters the AlGaInP layer from the surface of the wafer and prevents the Zn dopant atoms in the AlGaInP layer from activating.
In Electronics Letters, 12th March 1992, Vol.28, No.6, pp.585 to 587, in order to remove the hydrogen, the wafer is heated to a temperature of 450.degree.-740.degree. C. in a mixture of hydrogen and nitrogen after the growth of the p type AlGaInP cladding layer.
When this post-heat treatment is applied to the method of producing a semiconductor visible laser diode illustrated in FIGS. 9(a)-9(e), in order to effectively remove the hydrogen and increase the activation ratio of the Zn dopant atoms in the p type AlGaInP cladding layer 104, the post-heat treatment is carried out after the growth of the p type GaAs contact layer 107 shown in FIG. 9(e). However, if the wafer is heated to 450.degree..about.740.degree. C. in the mixture of hydrogen and nitrogen after the growth of the p type GaAs contact layer 107, As atoms unfavorably escape from the surface of the GaAs contact layer 107, resulting in a rough surface of the GaAs contact layer 107. The rough surface causes an imperfect connection between the contact layer 107 and the electrode 109, significantly reducing the reliability of the laser diode.
Meanwhile, in the above-described Journal of Crystal Growth article, an n type GaAs layer is grown on the p type AlGaInP layer to remove hydrogen from the AlGaInP layer and increase the carrier concentration in the AlGaInP layer. When this process is applied to the production method illustrated in FIGS. 9(a)-9(e), in order to effectively remove the hydrogen and increase the activation ratio of the Zn dopant atoms in the p type AlGaInP cladding layer 104, the n type GaAs layer is grown on the p type GaAs contact layer 107 shown in FIG. 9(e). After the wafer cooling process, the n type GaAs layer is removed. However, since it is difficult to selectively etch away the n type GaAs layer leaving the underlying p type GaAs layer 107 unetched, the p type GaAs layer 107 after the etching process has an uneven surface. The uneven surface causes an imperfect connection between the p type GaAs contact layer 107 and the electrode, significantly reducing the reliability of the laser diode.